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Solid State Amplifiers => Amplifier Discussion => Topic started by: bluesky6 on April 06, 2014, 04:52:47 PM

Title: Vox Pathfinder 10 mods
Post by: bluesky6 on April 06, 2014, 04:52:47 PM
Just got myself a Pathfinder 10 (the Pathfinder 15/R are going for way too much money for a mod platform).

After playing with it a little bit, I found that at high gain/distortion, there is a lot of speaker buzz on the little 6.5". No buzz with an external 12" cab. So I'm thinking of putting in a Jensen MOD6 to see if things improve. The Eminence 620h is tempting but costs more than the price of the amp...!

Question for those who have tried both the 15/R and 10: is it worth tweaking the 10 to recreate the pre-amp of the 15/R tone-wise? From the schematics, the two seem to have very similar gain and tone stages.

Other mods I'm thinking of:
- Change the gain resistor of the second stage to reduce the gain when the drive switch is on
- Use an analog taper pot for volume
- Zeners to "control" the supply voltage for the op-amps
- Remove/short the headphone resistors and use the headphone jack as a speaker out jack

Title: Re: Vox Pathfinder 10 mods
Post by: Roly on April 08, 2014, 09:43:48 AM
Hi bluesky6, welcome.



(http://profile.ultimate-guitar.com/profile_mojo_data/1/4/0/0/140027/pics/_c1181902_image_0.GIF)

A six inch speaker is worth replacing just because it's six inch, and a buzzy six inch speaker ...

Quote
- Change the gain resistor of the second stage to reduce the gain when the drive switch is on

That's R7 1Meg.  The overdrive from this stage is clipped by the following back-to-back red LED's.  You may find some tonal advantage in playing around with one of these LED's trying one with a different forward voltage (generally a different colour) with will give you asymmetric clipping of +ve and and -ve parts of the signal, which can produce a nice 2nd harmonic.

Quote
- Use an analog taper pot for volume

All pots are by their nature analog - I think you mean "audio" taper or log pot, which the volume control should already be.  Set it to 5/10 and measure the resistance from the wiper to each end.  If it's a linear taper then it will be about 25k to each end, but if it's quite different then it's already a log pot.

Quote
- Zeners to "control" the supply voltage for the op-amps

Do you have a reason/problem?  If you don't actually have a problem then I wouldn't bother, most op-amps have about 120dB of power supply rejection.

Quote
- Remove/short the headphone resistors and use the headphone jack as a speaker out jack

That should work, although you could always mount another socket somewhere for an external speaker and retain the headphone/Line Out option.

HTH
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 09, 2014, 09:51:44 AM
Roly, thanks for the suggestions and the schematic.

I'll look into replacing the leds, either with a single one of different color or with a couple of germanium diodes. There's a blog article where someone used a pair of 1N34As.

The volume control pot is a B50K which I assume means that it's linear as opposed to audio taper. An alternative to replacing it (since it's probably hard to get an exact physical replacement) is to replace R18 with a large value.

My problem is that of too much volume (hearing sensitive spouse and all that)  :-\

Other mods I'm thinking of:
1. Move the AC circuit from the main PCB and use a fuse holder instead. Will mean drilling a hole in the chassis. This will make removing the PCB easier for tweaks.
2. Remove all the original audio amp components and use one of those PSU+AMP TDA2030 boards from Eb*y. These boards include power rectifier and amp. The question is of available space...
3. Putting in sockets for the op-amps to try out the lower noise ones

I'm going to try to reduce the noise level as much as possible so that if I don't do the speaker out mod, I can use headphones without the hiss. I've a Vox AC30 Amplug and all I hear (with gain >2) is hiss.

Title: Re: Vox Pathfinder 10 mods
Post by: BigPolishJimmy on April 09, 2014, 10:07:26 AM
I've been curious about the pathfinder 10.  How do you like it so far?  Obviously you're looking to mod it, but is that more from dissatisfaction where it is lacking, or more from wanting to customize it to your liking?   

Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 09, 2014, 12:34:12 PM
I've been curious about the pathfinder 10.  How do you like it so far?  Obviously you're looking to mod it, but is that more from dissatisfaction where it is lacking, or more from wanting to customize it to your liking?

To be honest, I spent about 20 minutes playing through the amp before taking it apart. While the cleans were okay (I have a Fender FM65R as a comparison...!), I didn't like the drive channel, even using my external 1x12 cab. Hence the thought of reducing the gain on the 2nd op-amp. But that's my personal preference: I like a dry compressed crunch without too much fuzz.

I did find that, like some others, using the gain as some kind of volume control worked pretty well.

Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 09, 2014, 12:39:34 PM
Picture of the innards.

I don't like the incoming AC having to go through the main PCB.

The power amp on a separate PCB with a long 5-wire pigtail also looks nasty. The schematic Roly posted indicates a ferrite core but not on the amp that I have.

The big positive is the modability of the board. I have a Behringer GM108 (now for sale) which I wanted to mod but gave up on the idea because of all the SMT components.
Title: Re: Vox Pathfinder 10 mods
Post by: BigPolishJimmy on April 10, 2014, 09:31:55 AM
Thanks for the reply, and the pics.  Wow, I'm blown away at the how nice the board looks.  Honestly I'm surprised at that price point that there are serviceable components in there.  It really seems to lend itself to modification. 

I wonder if these will develop the hiss problems that the valvetronix series had? 

I'm going to have to go play one to see what's what about the sound.  I play mostly clean-ish and have a pedal for when I want distortion, and when I want distortion I want sustain for days and I get that from my pedal

Thanks again for posting this and humoring my threadjack

~Jim
Title: Re: Vox Pathfinder 10 mods
Post by: Roly on April 10, 2014, 02:46:40 PM
bluesky6

The trick with clipping diodes is to make them different, that's what gives you 2nd harmonic.  The LED's that are in there are clipping around +/- 2-3 volts.  A germanium diode will clip around 0.1V so you may have a problem getting enough signal into the output stage.

You can't assume anything from the taper code letter.  A fairly orderly system in the 1960's has been well corrupted by everybody baking their own cake.  These days the only way to be sure is the test I mentioned, but I'll be surprised if you don't find that whatever the code letter, it's already a log taper.

Trade spouse in on one that is a) deaf or b) mad for RnR.  My third one is both.  8|

1. Getting the mains off the PCB and well insulated is a very good idea, particularly if you want to tinker inside.   :dbtu:

2. that's not a "mod", that's a transplant.   :lmao:

3. there are lower noise op-amps than the 4550's, but you soon strike the law of diminishing returns and that you won't notice the difference between modern cheapies like LM833's, TL07x-series etc, and das zupervundendorfer's at $$$$ each.  If you can't hear it fully cranked with your ear against the speaker then making it 20dB quieter is pretty pointless.

- most hiss originates in the first stage

- reducing preamp hiss lower than the inherent hiss of the power amp is pointless.

Quote from: BigPolishJimmy
I wonder if these will develop the hiss problems that the valvetronix series had?

That's pretty unlikely in a fully solid-state amp.  Hiss generally comes from using a cheap op-amp in the first stage.  Spend an extra buck and quite large and lasting improvements can be made.


ed typotypo
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 10, 2014, 09:22:35 PM
I just ordered several sets of op-amps (2132, 2134 and tl072). One option I have for the amp is to use it primarily with headphones (due to aforementioned non-deaf spouse). So step one will be to see if I can reduce the noise level.

I have also been researching power transformers. Again, with the goal of reducing the volume, I was thinking of using a 10-0-10 transformer. This will "naturally" provide a lower voltage power supply to the op-amps without the voltage drop resistors. It will also reduce the max power of the audio amp. Or allow me to use a lower wattage and voltage transplant like a TDA7052A or the Velleman 3W amp kit.

I discovered several sets of transformers from Hammond that would just slot in to the existing mount holes.

Guess what: the compatible transformers range from 10 to 12VA power rating.

If the OEM transformer is of the same or similar capacity, that means the amp in factory configuration is really too underpowered to claim a 10W output, considering you really don't have 100% efficiency.

More thoughts to follow.

Roly, thanks for the pointers on the diodes. I'll start my experimentation with an green LED and may play with other types of diodes in series...
Title: Re: Vox Pathfinder 10 mods
Post by: Roly on April 10, 2014, 11:18:59 PM
Quote from: bluesky6
I have also been researching power transformers.

Sticking a tastefully chosen power resistor in series with the speaker is a lot cheaper.

Quote from: bluesky6
If the OEM transformer is of the same or similar capacity, that means the amp in factory configuration is really too underpowered to claim a 10W output, considering you really don't have 100% efficiency.

Power rating can generally be estimated fairly well by the size of the transformer core.

As a general rule an amp will consume a bit more than double it's power output, or put the other way, you aren't going to get more than about 40% of the input power as output.

Your amp doesn't deliver the output power claimed?  That would have to be a first.   8|

{Maybe I should take my battery powered "400 Watt" computer speakers back to the shop.}

Quote from: bluesky6
I'll start my experimentation with an green LED and may play with other types of diodes in series...

That's the spirit!  YMMV but others have had pleasing results.
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 13, 2014, 05:47:38 PM
Finally put back together the Pathfinder.

Mods include:
1. Green LED (see pic).
2. Jensen MOD 6-15 8ohm speaker

It was fun to play my usual power chord sequence while watching the LEDs. The green seems to clip earlier than the red: my interpretation from seeing the green LED come on earlier and brighter than the red. I can't speak to the improvement or lack-thereof from the LED change as I didn't spend that much time playing on the un-modded amp.

On the other hand, the Jensen speaker is a nice upgrade. Better, richer sound, and very nice chimey cleans. I can tell the difference from my Fender FM 65R.

I still don't like the following:
1. Jump in volume when switching from clean to drive
2. Usable volume when in drive is at the first notch. Yes, the volume control IS a linear taper pot. I measured it as suggested by Roly. An audio taper is on order

In drive mode, gain is good for crunch until 9am. After which it's fuzz overdrive. It's pretty similar to the FM 65R (dry crunch until 4, then fuzz overdrive...).

I'll be using the Pathfinder over the next week to see if the noise level is acceptable. It's definitely better than the Fender 15R (in the other thread): you really don't hear any hiss until you switch to the drive mode. If I don't like the noise level, I'll have questions about op-amp swap  ;)
Title: Re: Vox Pathfinder 10 mods
Post by: Roly on April 14, 2014, 02:55:41 PM
Ah, had a closer scrute...

First stage op-amp U1A uses the "gain" pot just as it suggests, it alters the gain by altering the proportion of Negative FeedBack (not the amount of signal fed forward at the Master Volume control).

In this case it may well actually require an anti-log pot if you want a smoother transition, or you can use a standard log wired (and working) backwards.

But I'd certainly give the simple audio/log path a go and see how it works.   :tu:
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 20, 2014, 05:06:31 PM
I managed to "finish" up the mods that I wanted to do on the Vox Pathfinder. It helped that the parts I ordered arrived.

Although not exactly the right parts: the audio taper pot was solid shaft instead of split/knurled. So I have to get a chicken head knob with a set screw in order to use the pot. The other option was to get the "right" pot from Eb*y with an unpredictable ETA.

The other new parts were a bunch of different op-amps. Woohoo!

So changes I made included:
1. Swap out the linear taper volume control for the audio taper. I instantly had a better, more usable volume range. Otherwise, usable volume = first mark on the dial.
2. Soldered a 1M resistor across R7 to half the effective resistance hence reducing the gain with Drive. The theory was to have Gain control with more usable crunch range before fuzz. Result: not really effective.
3. Desoldered the 4558DD op-amps and put in sockets. For newbies considering modding their amps: desoldering wick is your friend
4. I replaced U1 with an OPA2164 and U2 with an NE5532. The latter because U2A was just a post-tone pre-amp so low-noise and clean was important. The former because I'd used the same chip to replace a TL072 on the Fender 15R and I got a "better" crunch from that swap.

I don't have an experience enough ear to really tell the benefits of the op-amp swap. At least, the differences weren't as obvious and I didn't spend any time doing A/B testing before I put everything back together again. But the swap didn't hurt.

I'll use the amp over the next week to see how much more I like it.
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on April 25, 2014, 09:41:15 PM
And... Eureka!

One of the things that folks don't like about the Vox Pathfinder is the usability of the drive "channel". Turn it on and you more distortion than overdrive. To the point where, if Google is fact, the "LED mod" is the most popular mod on Pathfinders: where the LEDs that are used to clip or distort the signal, are removed.

I don't like the default distortion effect and have been looking for a middle ground: to have more adjustment in the gain control between high gain clean, crunch, and distortion.

I think I have found my middle ground. YMMV.

The combination is the following:
1. Swap the volume control for an audio taper pot, replacing the factory linear taper. "Part is Parts" has the right one that will fit the factory chicken head knob.
2. Solder a 1M resistor in parallel with R7 to reduce the gain in the second stage pre-amp. This gives more adjustability to the gain pot.
3. Increase the clipping voltage of the LED pair.

For #3, LEDs apparently clip at 1.6V or more (I've not managed to measure myself, but if Google is fact...). So the trick there, if you want to have more adjustability in the gain control and therefore more headroom, is to add more diodes in series with the LEDs to increase the clipping voltage.

My choice was a pair of germanium diodes which I got from Frys (part number NTE110MP). They had about 230+mV (measured) in clipping voltage. So putting them in series would increase the clipping voltage by that much. For even more headroom, one could use another set of diodes in series or a silicon diode.

Anyway, in addition to the germanium diodes, I added in a 220pF cap in parallel. This is, in theory, supposed to removed some of the high frequency fuzz. Not really noticeable, to my old ears, but the 220pF value was chosen because I thought the original 100pF I used didn't work that well.

Finally, I added a 100ohm resistor in series with the LED+diodes+cap. This is supposed to mellow the effect.

The outcome of all these changes is a much more usable drive mode. More crunch and a very marked reduction in fuzz and fizz.

Pics of the mod are attached.

IMHO, this is better than removing the LEDs all together as you still have an option to go all out in metal distortion mode.

I'll be putting everything back together again and using this over the next few days.

EDIT: Put in lower res pics
Title: Re: Vox Pathfinder 10 mods
Post by: Roly on April 26, 2014, 05:07:57 AM
"Eureka!"  indeed.  Well done.  Thanks for the report.   :dbtu:

I'll just note that it has been found generally with diode signal clippers that a more musical result can be had by using asymmetric clipping, either different colour LED's (that have different cut in voltages) or more diodes in one leg than the other, giving the same effect.  This produces more even-order harmonics.
Title: Re: Vox Pathfinder 10 mods
Post by: bluesky6 on May 11, 2014, 10:59:32 PM
Here's a schematic of the mod.

I thought it was easier to hand-draw and scan than to learn LTSpice or some other "modern" tool.   xP

GE means Germanium diode.
Title: Re: Vox Pathfinder 10 mods
Post by: ehsan_zt on March 24, 2016, 04:32:50 PM
Hi, sry for bumping this topic, but I have some questions regarding these mods, and if I wanted to post a new topic I had to reference to this topic more than once :loco

It's little confusing, does paralleling a 1M resistor to R7 has any effect or not ? (reducing gain when we are in overdrive channel)
Pot labeles on my amp are: B104 for gain,ALPHA B50Kohm for treble, A50Kohm for bass and ALPHA B50Kohm for volume. I think I need a log taper for volume,and since I can not find any log pots where I live, can I swap bass pot with volume pot ? (won't there be any problems with bass ? )
Should I use anti log pot for gain ? if so,how can I change a linear pot to anti log ?
Title: Re: Vox Pathfinder 10 mods
Post by: blackcorvo on January 27, 2018, 03:15:10 PM
Answering these, as I have some things to add to this topic. Thought it was better than making a whole new topic.

It's little confusing, does paralleling a 1M resistor to R7 has any effect or not ? (reducing gain when we are in overdrive channel)

Paralleling resistors makes the value lower. When they're the same value, you simply divide the value of the resistors by the number of resistors. So, 2 1M resistors would be, considering 1M as being 1000k ohms, 1000k / 2 = 500k.

Pot labeles on my amp are: B104 for gain,ALPHA B50Kohm for treble, A50Kohm for bass and ALPHA B50Kohm for volume. I think I need a log taper for volume,and since I can not find any log pots where I live, can I swap bass pot with volume pot ? (won't there be any problems with bass ? )

Yes, if you swap the Bass and Volume pots around you'll have better control over the Volume, but then you might not have great control over the Bass (or at least not as gradual). Try it out, worst case you'll have to swap the pots back.

Should I use anti log pot for gain ? if so,how can I change a linear pot to anti log ?

That's not necessary, the gain pot is pretty responsive as it is.

---------------------------------------------------------------------------------------------------------------------

So, besides the responses, I just wanted to add to this topic by mentioning some mods I've done to my own Vox Pathfinder 10.

Here's the schematic with the mods: https://drive.google.com/open?id=1cmLku3r3WP6RrpEsWeMjAIwxlQHuqlLA

So, what these mods have done for me:
- Changed C6 from 220n to 470n to up the bass response a bit, since I like using a treble booster with this amp;
- Removed the Red LEDs from the board and placed 2x White LEDs soldered directly onto R7, for a less harsh distortion even on "clean";
- Changed R8 to 100k, to get a bit more gain on "clean", which coupled with the White LEDs, sounds sweet;
- P3 was actually not a change due to seeking better tone, I bought this amp used and the Bass knob wasn't working, so I changed it with what I had at the time and never changed it back;
- Reducing R5 is just an alternative/suggestion to increasing R8, which I haven't tried (yet).

And here's a few sound samples of slightly different configurations. The guitar I'm using is a cheap SG copy with it's original pickups, and a self-made "Brian May Treble Booster" pushing the amp, all with the amp's overdrive switch On:

https://drive.google.com/open?id=1U020qi_7YmRRCM21xxvB2zgSsClibI6O
https://drive.google.com/open?id=1ztU0C0BBcWrmXaOP1azp4cRmpcetUanL
https://drive.google.com/open?id=1hApyLMedKybXe9PxBCcmxcbIyK1f-xOU

I'm really loving how it sounds now! Even without the treble booster, it sounds pretty sweet!
Title: Re: Vox Pathfinder 10 mods
Post by: j_flanders on January 30, 2018, 05:41:43 AM
Here's the correct schematic by the way. (Edit: Sorry about the large pictures. There's a scrollbar at the bottom of the post to see the right most part of them.)
Green means 'correct component value', red arrow means 'the component was in the wrong place' or 'incorrectly connected'.

One correction not included is about the note: "Gain switch shown in Clean position"
The schematic shows one part of the switch in clean position, namely the diodes taken out of the circuit, but the other part shows it in distorted position, namely R4, which should not be connected to ground for Clean.

(http://i65.tinypic.com/15qrsky.jpg)

Here's the circuit board tracing.
Red is signal, black is ground, grey is power supply to opamps. White components are resistors, greens are capacitors.
I was only interested in the preamp section, so I stopped at C19, just before the power amp section.
(http://oi68.tinypic.com/eip8ad.jpg)

I love this little amp. I've been playing it daily (nightly actually) for the past two years or so.
The clean channel is perfect, the overdrive channel not so much. I tried a ton of mods all over the preamp section. I'll post about them later on.
Title: Re: Vox Pathfinder 10 mods
Post by: blackcorvo on January 30, 2018, 05:57:27 AM
Here's the correct schematic by the way.

Thank you for the revised schematic! I didn't even notice the drive switch position thing until you pointed out. It's also wrong in my modified schematic... whoops!

I'm now thinking about possibly adding an FX loop to this amp, but I'm still debating how I should approach it.
Title: Re: Vox Pathfinder 10 mods
Post by: j_flanders on January 30, 2018, 06:59:44 AM
So, what these mods have done for me:
- Changed C6 from 220n to 470n to up the bass response a bit
Must be only a tiny bit as C6 (220nF) is in series with C5 (4,7nF).
Caps in series are like resistors in parallel, so the total capacitance of stock C6 and C5 is 4,601nF.
Changing C6 from 220nF to 470nF would result in 4,653nF, a change of 53pF or a 1% increase...

Now, C5 is in parallel with the upper part of the gain pot, so in reality things are a bit more complex and variable:
When the gain pot is maxed at 10, C5 is effectively shorted and only C6 is active. Turning down the gain slowly brings C5 into play, putting it in series with C6, lowering the capacitance, increasing the low cut / high boost.

Your mod will have most effect when gain is high and least effect when gain is low.

Changing C5 instead of C6 could give you more reach/control at lower gain settings because stock, it is a much lower value than C6.

Removed the Red LEDs from the board and placed 2x White LEDs soldered directly onto R7, for a less harsh distortion even on "clean";
It's still 'hard clipping' to ground, as these go to the inverting pin, but since they are in parallel with the feedback resistor R7 and cap C7, I do agree that it sounds a little less harsh. They will clip in clean mode, but you will have to turn up the gain considerably and by then you're also bosting lows a lot more, resulting in a flatter pre-clipping frequency response, which is a large part of the reason it sounds less harsh, more balanced.

Those green wires soldered to the back of the board you see on the PCB tracing picture was a similar mod, me trying different diodes, resistors and caps in the feedback loop.
Title: Re: Vox Pathfinder 10 mods
Post by: blackcorvo on January 30, 2018, 07:32:05 AM
j_flanders, I have only a crude understanding on filters, but in my simple terms, what I see with C5 is a high-pass filter, not a coupling capacitor.
The difference lies in the resistor (potentiometer in this case) in parallel with the capacitor, as it "leaks" the frequencies that can't get throught C5. What I did by changing C6 was allow more of these lower frequencies throught to the next stage.

About the clipping, the amp came modded with a Si+Ge clipper, so poorly added in there that the copper foil was coming off the board. I originally put it back to the stock red LEDs, but after having connection issues, I added screw-in terminals to that point of the board for testing. They're still there, in case I wanna mess with that in the future.
I decided for the soft clipper because I don't like how the op-amps sound overdriven. There's a harsh change between clean and driven tone I don't like, and using white LEDs (which take like 3v to conduct) that harshness is reduced a lot. I haven't tried mixing a hard clipper in there yet, but I don't feel it's necessary at this moment.
Title: Re: Vox Pathfinder 10 mods
Post by: ehsan_zt on March 05, 2018, 02:39:23 PM
Thanks blackcorvo for answering my questions. I'm tempted to try some of your suggestions.

I too have some things to add:
1- I've been mainly experimenting with drive channel and trying different diodes,I've made a topic over diystompboxes :
http://www.diystompboxes.com/smfforum/index.php?topic=114013.msg1057967#msg1057967
I've found out the most effective way to make drive channel better, without altering too many things is diode limiter (check above link). You can get a nice transition from crunch to distortion.

2- Another interesting thing I found out was this video:
https://www.youtube.com/watch?v=rRAv76ylAcI
what this guy did is he simply removed C12 2A274K!
I tried this,and it's AMAZING ! when you remove C12, amp will sound clearer and there is an increase in volume. I really suggest trying this and removing it or replacing it with something lower (like 2A272K).
I'd love to know the science behind it,if some one can explain?!

3- I changed my volume pot(B50K) with my bass pot(A50K),now volume is more versatile specially when in drive mode. It's a simple effective tweak. :dbtu:
Title: Re: Vox Pathfinder 10 mods
Post by: blackcorvo on April 11, 2018, 08:16:09 AM
2- Another interesting thing I found out was this video:
https://www.youtube.com/watch?v=rRAv76ylAcI
what this guy did is he simply removed C12 2A274K!
I tried this,and it's AMAZING ! when you remove C12, amp will sound clearer and there is an increase in volume. I really suggest trying this and removing it or replacing it with something lower (like 2A272K).
I'd love to know the science behind it,if some one can explain?!

C12 is part of a filter on the second op-amp. It goes in parallel with the feedback resistor R15, but isn't completely across the feedback loop because of R17.
I believe it's a low-pass filter (again, I'm not super well versed in filters), which would explain the change in volume. I'd guess it's there to remove some of the harshness coming from the preamp, and possibly to limit the volume some before it hits the power amp.

Personally, I'd rather play around with the filter instead of just removing that capacitor. You might be able to emulate a cab if you do some clever use of that op-amp. Perhaps copy the values from something like RunOffGroove's Thor output circuit? Or more appropriately, their Britannia circuit? Food for thought.
Title: Re: Vox Pathfinder 10 mods
Post by: blackcorvo on April 22, 2019, 02:20:12 AM
j_flanders, thanks again for the photo of the board. I decided to do a simple diagram on it of how to do the mod. I'll leave the coupling cap and bass pot I changed out of it, as I don't think it changes much in regards to the overall response of the amp.
I decided to recommend removing R8 along with the Red LEDs, as I think it muffles the amp too much.

https://imgur.com/qZBD5JF
Title: Re: Vox Pathfinder 10 mods
Post by: phatt on April 26, 2019, 04:05:31 AM
j_flanders, I have only a crude understanding on filters, but in my simple terms, what I see with C5 is a high-pass filter, not a coupling capacitor.
The difference lies in the resistor (potentiometer in this case) in parallel with the capacitor, as it "leaks" the frequencies that can't get throught C5. What I did by changing C6 was allow more of these lower frequencies throught to the next stage.


Correct,, C5 is almost irrelevant for bass response going to U2, C5 is just a treble bleed for lower gain settings. C6 sets the low freq roll off point that gets to the next stage.
Phil. :tu: